Dissociated cell cultures of dopaminergic neurons from embryonic rat mesencephalon were grown in completely defined serum-free medium. Changing the medium of these cultures with medium containing 4.0 mM glutamine leads to complete destruction of the cultures via the NMDA- type of glutamate receptors. A low level of glutamate (~M) contaminating the glutamine initiates damage; over the next 3-6 h the cells are damaged with an accompanying increase in the level of glutamate in the extracellular medium. Most of this glutamate is generated extracellularly since it also appears if the medium is removed and incubated with no cells. Most likely, damaged cells release mitochondria (with the enzyme glutaminase) into the culture medium which converts the extracellular glutamine to glutamate.A stimulus of physiological significance is also responsible for culture damage: high levels of potassium. This damage also causes extracellular generation of glutamate and is blocked by APV. Further damage would result since once mitochondria are released by cells, they no longer provide energy (ATP) to these cells leading to energy failure. A model of ischemic damage is proposed: in a compromised brain, excess potassium would damage neurons via NMDA-receptors; the damaged cells would release mitochondria which would produce glutamate from the abundant glutamine in the cerebrospinal fluid; later, reperfusion of the area would introduce fresh glutamine and further production of glutamate and neuronal death. Regions of the brain with high susceptibility to ischemic damage would not necessarily contain a higher concentration of NMDA receptors but may contain NMDA receptors clustered in mitochondria-rich areas (terminals).